1. Field of the Invention
This invention relates to the field of fluid powered actuators, and in particular to an actuator for ejecting stores (e.g. bombs or missiles) from an aircraft.
2. Description of the Prior Art
Ejector systems for ejecting stores from aircraft have a long history of use. The ejector system sends a pressurized fluid, either gas or liquid, to an actuator. The actuator extends and pushes a store from the aircraft. The actuator needs to push with such force that the store rapidly moves through the airflow streaming over the aircraft. Less than rapid ejection of the store results in the store's trajectory being changed by the airflow. Any changes in the store's trajectory result in inaccurate landing of the store or the store colliding with another store or with the aircraft.
The pressurized fluid that ejector systems use to extend the actuator include cold gases, hot gases, or hydraulics. An example of a cold gas ejector system is disclosed in U.S. Pat. No. 5,583,312 to Jakubowski entitled "Cold Gas Ejector Rack" ("Jakubowski '312"). An example of a hot gas ejector system is disclosed in U.S. Pat. No. 5,029,776 to Jakubowski et al entitled "Variable Explosive Source for an Ejector System"("Jakubowski '776"). All of these U.S. Patents are expressly incorporated herein by reference in their entireties.
The actuators used in ejection systems typically comprise piston assemblies. Examples of piston assemblies are disclosed in previously referenced Jakubowski '312 and Jakubowski '776, along with U.S. Pat. Nos. 4,388,853 to Griffin et al. entitled "Missile Launchers" ("Griffin") and 4,088,287 to Hasquenoph et al. entitled "Dual-Purpose Ejector For Aircraft Load Jettisoning Mechanism" ("Hasquenoph"), both of which are expressly incorporated herein by reference in their entireties.
Telescoping piston assemblies are used in actuators to save space. Telescoping pistons take up less volume when nested in the pre-stroke position. Having piston assemblies take up less volume is desirable due to the space limitations aboard aircraft. Previously referenced Jakubowski '776, Griffin, and Hasquenoph disclose telescoping piston assemblies.
Referring now to PRIOR ART FIGS. 1-3, wherein like reference numbers refer to like elements in the figures, a conventional actuator 10 has a cylindrical housing 12 and nesting, telescoping pistons 14. The housing 12 has a base 16 and a wall 18. The wall extends from the base 16 and terminates in an open end 20. The wall 18 has an interior surface 22. A stopping flange 24 radially extends into the housing 12 from the open end 20.
The telescoping pistons 14 have an outer piston 26 and an inner piston 28. The outer piston 26 has a flanged first end 30 that is proximate to the base 16, and a flanged second end 32 that is distal to the base. The first end 30 is slidably sealed against the housing interior surface 22 via seals 34 disposed therebetween. This arrangement permits the outer piston 26 to extend from the housing 12 until the first end 30 contacts the stopping flange 24, while inhibiting fluid transfer past the seals 34.
The inner piston 28 has a flanged open end 36 proximate to the base 16, and a closed end 38 distal to the base. The inner piston 28 is slidably sealed against the outer piston 26 via seals 40 disposed between the outer piston 26 and the inner piston open end 36. This arrangement permits the inner piston 28 to extend from the outer piston 26 until the inner piston open end 36 contacts the outer piston second end 32. Other conventional telescoping pistons may have more than two pistons.
The telescoping pistons 14 extend from the actuator 10 in a stroke to push the stores from the aircraft. The telescoping pistons 14, as shown in PRIOR ART FIG. 1, are nested in the housing 12 in the pre-stroke position 42. PRIOR ART FIG. 2 shows the pistons 14 stroked out to the point of staging position 44. At staging position 44, the outer piston 26 is fully extended from the housing 12 with the outer piston first end 30 in contact with the stopping flange 24. The inner piston 28 is still nested in the outer piston 26. PRIOR ART FIG. 3 show the pistons 14 at the end of the stroke position 46. The inner piston 28 is fully extended from the outer piston 26, with the inner piston open end 36 being in contact with the outer piston second end 32. As a result of the nesting arrangement, the stroke length of the pistons 14 are essentially twice the length of a single piston.
The conventional telescoping pistons 14 have a stepped decrease in force during the stroke. This is a result of the acting pressure area decreasing in a step-wise fashion during the stroke. Between the pre-stroke position 42 and the staging position 44, the acting pressure area is a first area 48 defined by the circumference of the outer piston first end 30. After the stage 44, the acting pressure area steps down to a smaller, second area 50 defined by circumference of the inner piston open end 36. Referring now to FIG. 4, a graph 52 shows the force vs time plot 54 of a typical telescoping piston arrangement. The instantaneous drop in force at point 56 corresponds to when the pistons 14 reach the staging position 44 and the acting pressure area drops from the larger, first area 48 to the smaller, second area 50.
A stepped decrease in force during the stroke is not desirable under some circumstances. The stepped decrease in force during a stroke results in a relatively high peak ejection force. The relatively high peak ejection force results in requiring a supporting frame, such as an airplane wing, that is reinforced enough to accommodate the relatively high peak ejection force. The reinforced supporting frame adds excess structure and weight to the aircraft, with accompanying undesirable results as is known to those skilled in the art.
What is needed, therefore, are telescoping pistons that have a relatively constant net pressure area during the entire stroke to provide the requisite work while having a relatively low peak ejection force.